Some injection molding machines use an induction coil to melt material in a boat before injecting the material into a mold. In horizontally disposed injecting molding machines, for example, material can be melted in a boat that is positioned for horizontal injection. Some machines have utilized a boat that is substantially U-shaped; that is, a boat that includes body with a base and side walls extending partially upwardly therefrom but that end around a midpoint or equator. This configuration results in a low-walled vessel design that resembles a partial portion (e.g., lower half) of a tube (as opposed to a fully enclosed, round tube) with an open top portion designed for exposure to a magnetic field from the induction coil in order to melt material therein. This low-walled boat design can reduce both boat and plunger tip life. Also, the U-shaped design is subject to molten metal to flow over its sides during melting or during plunging of the metal. Further, because the plunger tip is minimally captured on top, it has some play in the direction perpendicular to the bore, which can result in it digging into a lip of the walls or cutout region, causing wear. Poor control of a plunger-tip to boat wall gap can allow penetration of flash into a gap that is too large on the bottom or sides of the tip during injection. Also, metal flash can build up at an edge of the cutout region in such a U-shaped boat. The boat may be unstable and have a greater tendency to flex. Moreover, heating in a U-shaped boat utilizes primary and secondary fields from an induction coil; such boat designs can suffer from excessive heating at its top edges, causing the boat to expand and curve if cooling is insufficient.
In some skull melting machines, a vertically positioned concentrator-type cage melter, surrounded by an induction coil, can be used to melt materials. Skull melters may have a vertically enclosed tubular configuration, for example, or may have a number of segments or fingers positioned in a substantially circular or tubular configuration, having multiple slots or openings therebetween, connected to a solid bottom, for example. FIGS. 8, 9, and 10 show a cross-sectional side view, a top view, and a plan view of an exemplary skull melter, as known in the art. This type of skull melter can create a semi-levitated melt. The intense stirring generated by the magnetic field in the skull melter fuses any high melting temperature charge components and homogenizes the melt in both chemical composition and temperature. This can translate into faster melt down, better mold filling (for castings) and a higher quality product.
When melting materials in an injection molding system, uniform temperatures in ranges appropriate to the meltable material should be implemented and maintained in order to produce quality molded parts. Utilizing effective vessels during melting can improve such quality.